This invention relates to network controlled converters in general and more particularly to an improved method and device for determining a reference voltage synchronous with the network for a control set of network controlled converter, especially in an a-c network having harmonics, after a network disturbance.
A device for smoothing vector component voltages with the correct phase is described in German Offenlegungsschrift No. 20 19 263. In the disclosed device, two voltages are visualized as orthogonal components of a vector in an orthogonal reference system fixed in space and fed to the vector input of a vector rotator. Two normalized voltages which, as the sine and cosine of a variable angle, correspond to the Cartesian coordinates fixed in space of a unit vector rotating at a transformation frequency are fed to the angle signal input of the vector rotator. These normalized voltages serve as transformation elements for a coordinate transformation so that the vector rotator supplies the orthogonal components of the vector in a reference system rotating synchronously with the unit vector at its output. The quotient of these rotating orthogonal components corresponds to the tangent of the angle coordinate of the transformed vector and is fed to a controller, the output signal of which is entered as a frequency proportional control voltage into a sine and cosine generator for forming the normalized voltages for the angle signal input of the vector rotator. As a result, the rotating unit vector is slaved to the given vector in such a manner that the angle coordinate in the rotating reference system becomes zero. The transformed component of the vector which is parallel to the unit vector, is fed to a further controller, the output signal of which represents the amplitude of the vector when the angle coordinate is adjusted. Through multiplication of this amplitude by the outputs of the sine-cosine function generator, the desired smoothed orthogonal vector components fixed in space are then produced.
In unpublished German patent application P No. 33 03 454.0, this method is expanded for a low noise frequency measurement on a multiphase line system for the transmission of electric power. If, for instance, the frequency of an a-c voltage network is to be determined, two voltage signals which determine an actual vector describing the network voltage system in a reference system fixed in sapce are formed from measurement values. These vector components fixed in space are converted by a frequency transformation which is performed by the mentioned vector rotator, into two frequency transformed, orthogonal, slaved system components. The frequency of the transformation elements, i.e., the frequency of the unit vector fed in at the angle signal input of the vector rotator, is now determined in that a reference value is given for the angle coordinate of the transformed vector, and the control deviation of the angle coordinate from this reference value is leveled out. In this arrangement, anticipatory control for the frequency of the transformation elements with the nominal frequency of the a-c network which represents the approximate mean value of the network frequency is supplied. As long as the a-c voltage network is present, the frequency of the variable frequency transformation elements corresponds, in the balanced condition, to the desired network frequency so that the oscillation frequency is taken off directly as the desired frequency measurement value at the frequency control input of the sine-cosine generator.
In network controlled converter systems, for instance, in converters for feeding into a given voltage network and, in particular, for feeding into a high voltage d-c transmission system, it is frequently required that the energy transport which is interrupted during a disturbance of the network by a blockage of the converter be resumed as fast as possible after the distrubance has ended. A condition for the restarting of such a network controlled converter system is that a correct phase of reference voltage be available, if possible, immediately for synchronizing the control unit of this system. In these applications, therefore, not only the frequency but also a rapid and correct determination of the phase of the network is required.
The recurring voltage, however, consists not only of a suddenly switched on fundamental. In addition, harmonics with unknown frequency as well as transients with different unknown frequencies and decaying amplitudes occur, where these harmonic components represent an asymmetrically decaying system. These harmonics are not necessarily integral multiples of the network frequency, even if they begin as a rule with the second harmonic and frequently contain dominant harmonic oscillations of the third and higher order.
It is, thus, an object of the present invention to describe a method and a device which permit making available a network sychronized reference voltage by which the control unit of a network controlled converter can be controlled without the danger of commutation faults, as soon as possible after the end of a network disturbance.